Fo"cal (?), a. [Cf.F. focal. See Focus.]
Belonging to,or concerning, a focus; as, a focal point.
Focal distance, or length,of a lens or mirror Opt., the distance of the focus from the surface of the lens or mirror, or more exactly, in the case of a lens, from its optical center. --Focal distance of a telescope, the distance of the image of an object from the object glass.
© Webster 1913.